1. Field of the Invention
The present invention relates to a method of increasing capacity in a fixed wireless access communications network. The invention also relates to a computer system for implementing the method, a computer program for controlling the computer system and a communications network deployed using the method.
2. Description of the Prior Art
Fixed wireless access communications networks use a fixed antenna at each subscriber premises where the antenna is typically directional. Each subscriber antenna communicates with a base station (to which it is directed, in the case that directional antennas are used). Each base station is in turn connected to a physical communications network such as a public switched telephone network via a transmission link. A typical base station supports many subscribers, for example 500 to 2000 subscribers in the IONICA (trade mark) system. The resulting fixed wireless access system is capable of delivering a wide range of access services such as POTS (public operator telephone spice), ISDN and broadband data.
When a fixed wireless access telecommunications system is initially deployed, a base station of a particular capacity is installed to cover a particular populated area. The capabilities of the base station are designed to be commensurate with the anticipated coverage and capacity requirement Subscribers"" antennas are mounted on a building, for instance, on a chimney, and upon installation are normally directed towards the nearest (or best signal strength) base station or repeater antenna (any future reference to a base station shall be taken to include a repeater).
In order to meet the capacity demand, within an available frequency band allocation, fixed wireless access systems divide a geographic area to be covered into cells. Within each cell is a base station with which the subscribers"" stations communicate; the distance between the cells re-using the same frequency being determined such that co-channel interference is maintained at a tolerable level. When the antenna on the subscriber premises is installed, an optimal direction for the antenna is identified using monitoring equipment The antenna is then mounted so that it is positioned towards the optimal direction.
As already mentioned, fixed wireless access systems divide a geographic area to be covered into cells. For initial planning and design purposes these cells may be represented as hexagons, each cell being served by a base station (in the centre of the hexagon) with which a plurality of subscriber stations within the cell (hexagon) communicate. When detailed cell planning is performed the ideal hexagonal arrangement can start to break down due to site constraints or for radio propagation reasons. The number of subscriber stations which can be supported within each cell is limited by the available number of carrier frequencies and the number of channels per frequency.
Base stations are expensive, and require extensive effort in obtaining planning permission for their erection. In some areas, suitable base station sites may not be available. One aim in the design of a fixed wireless access system design is to have as few base stations as possible, whilst supporting as many subscriber stations as possible. This helps to reduce the cost per subscriber in a fixed wireless access system. Another aim is to increase the traffic carrying capacity of base stations whilst at the same time keeping interference levels within acceptable bounds. This is referred to as trying to optimise or increase the carrier to interference level ratio. By increasing the traffic capacity the number of lost or blocked calls is reduced and call quality can be improved. (A lost call is a call attempt that fails.)
Cells are typically grouped in clusters as shown in FIG. 1. In this example, a cluster of seven cells is shown. Within each cluster 7xc3x976=42 frequencies are each used once. The term xe2x80x9cfrequency re-use factorxe2x80x9d is used to refer to the number of sets of frequencies that the total number of available frequencies is divided into. In this case, the frequencies are divided into 7 sets, one for each cell in a cluster.
FIG. 2 shows how a larger geographical area can be covered by re-using frequencies. In FIG. 2 each frequency is used twice, once in each cluster. The frequency re-use factor N is 7. Co-channel interference could occur between cells using the same frequencies and needs to be guarded against through cell planning.
When the capacity of a cell or cluster is exhausted one possibility is to sectorize each cell. This involves using directional antennas on the base station rather than omnidirectional antennas. The 380xc2x0 range around the base station is divided up into a number of sectors and bearers are allocated to each sector. In this way more bearers can be added whilst keeping interference down by only using certain frequencies in certain directions or sectors. The frequency re-use factor is a product of the base re-use factor and the sector re-use factor.
Known approaches for seeking to increase system capacity include fixed frequency planning (FFP) which involves carefully planning reuse patterns and creating sector designs in order to reduce the likelihood of interference. For example, FIG. 3 shows an example of a fixed frequency plan with a frequency reuse factor N of 4 and which is known as the xe2x80x9cmirror methodxe2x80x9d. Each sector with the same reference numeral is constrained to use a specific set of frequencies that are different from the frequencies used by sectors with different reference numerals. There are four different reference numerals 31, 32, 33, 34, for each of four different frequency sets. The letters H and V are used to denote horizontal and vertically polarised frequency channels. However, fixed frequency planning is problematic because it is often difficult to map a frequency plan onto an actual communications network. This mapping process is complex, time consuming and adds to costs.
Further, when a communications network is deployed according to a fixed frequency plan, the base station locations and pattern of frequencies used for the communications links needs to adhere to the fixed frequency plan closely, in order for the benefits of the fixed frequency plan to be achieved.
The deployment of base stations in a fixed wireless access network is conducted such that anticipated use within a cell can be accommodated. Nevertheless, in some markets subscriber demand has far exceeded predictions. Further, the increasing use of the intemet has changed subscriber call characteristics considerably since subscribers will be connected to intemet service providers for extended periods of timexe2x80x94even if there is little flow of data. Difficulties arise in the maximisation of capacity whilst at the same time maintaining specified link performance levels for all subscribers. Whilst the frequency plans aim to reduce interference, specified link performance levels are not necessarily maintained for all individual subscribers. As subscribers are added to the fixed frequency planned network, there are knock-on effects on the link performance provided to other subscribers. Accordingly there is a requirement to increase the capacity of fixed wireless access networks.
The installation of excess base station capacity at the start of a network rollout has a serious problem in that it requires an expensive up-front investment in extra network infrastructure by the operatorxe2x80x94and this infrastructure will take a considerable time to produce any return on the investment There is also a risk that this extra capacity may never be used, since the prediction of service take up by potential subscribers is not an exact science, and market conditions can change unpredictably. This approach amounts to a serious financial risk, that few operators wish to accept.
The capacity of the network can be increased by fitting extra equipment at the base stations. This technique has its limits, however, determined by the fundamental design of the equipment concerned, and also the amount of frequency spectrum available. Once this limit has been reached, the only possible way of providing further capacity (within the existing frequency plan) is to install additional base stations at new cell sites, interspersed between the existing sites.
One problem with adding cell sites to an existing network is that the new base stations will cause interference to existing subscriber equipment located in their vicinity. Since FWA systems do not include roaming (roaming is a feature essential in mobile telephones networks, which allows a subscriber equipment to change the base station it is using automatically, is not provided) because the directional antennas used at the subscriber equipment to improve the radio performance, makes its inclusion impractical within reasonable cost limits, those subscriber links suffering loss of service due to interference require a manual change of host base station, which involves repointing the subscriber equipment towards the new base station. This process requires suitably trained personnel travelling to customer sites, and with potentially hundreds of subscribers involved, this takes a significant time period, during which affected subscribers have lost service due to the interference. Network operators have generally regarded TDMA/FDMA FWA networks using directional subscriber equipment antennas, as not expandable due to this loss of service. Obviously, this may result in a serious curb on the growth of their business.
The present invention seeks to provide a method and apparatus for increasing the capacity of a fixed wireless access network. In particular, the invention seeks to increase the capacity without loss of service to existing subscribers.
In accordance with a first aspect of the invention, there is provided a method of increasing capacity in a fixed wireless access network comprising the steps of installing an additional base station within an existing cellular deployment plan; incrementally increasing base station transmitter power levels from an initial level too low to cause interference to subscriber terminals within an area of coverage of the additional base station until normal operating signal power level is achieved; at each incremental power level measuring interference levels caused by the additional base station at each subscriber terminal within its coverage area to ensure that transfer of service at affected subscriber terminals from their existing base station to the additional base station may be undertaken before the interference levels become service affecting; and reducing power levels of the existing base stations according to the new cell boundaries.
In accordance with a further aspect of the invention, there is provided a method of increasing capacity in a fixed wireless access network wherein subscribers have directional antennas, comprising the steps of installing an additional base station within an existing cellular deployment plan; incrementally increasing signal power levels from an initial level too low to cause interference to subscriber receivers within an area of coverage of the additional base station until normal operating signal power level is achieved; determining for each power level, at each subscriber premises within the area of coverage of the additional base station, the signal level from additional base station and comparing the signal relative to a base station with which the subscriber has been communicating; and repointing the subscriber antenna if the signal level is improved.
The invention thus minimises the loss of service to existing customers during a capacity upgrade for a fixed wireless access system. It is possible, using low powers to predict which subscribers may be repointed. The concept of reducing service outage is typically regarded by operators as a higher priority than minimising the time taken or the amount of resource used in order to provide seamless service transition when a subscriber""s antenna is reoriented. The present invention thereby enables a minimum of visits by personnel in connection with this process, for RSS repointing.
Preferably, prior to the installation of an additional base station, the existing network is characterised and optimised. A new frequency plan may be deployed for the existing network such that there are spare radio channels available for the additional base station to use. There is a need for spare radio channels for the new cell site to use in some cases these may be found within the existing frequency plan.
The incremental increase in signal level can be predetermined. The signals for which the power levels are monitored can be characteristic signals such as control channel signals. The process of increasing signal power levels can also include an adjustment of antenna down-tilt. The normal operating signal power level can be determined by a balance in traffic load across new and old base stations.
In accordance with the present invention there is provided a method of increasing capacity in a fixed wireless access network wherein subscribers communicate with base stations via directional antennas, comprising the steps of:
i) installing an additional base station;
ii) transmitting signals from the additional base station such that interference is not suffered by existing subscriber stations;
iii) at subscriber stations within an area of coverage of the additional base station measuring received signal levels from the additional base station and comparing with the signals received from previously installed base stations;
iv) selecting a level of output power for the additional base station power level may be increased whereby to cause interference to those subscribers which are most susceptible to interference;
v) repositioning the antennas of subscribers most susceptible to interference towards the additional bas station;
vi) increasing the output power of the additional base station and repeating step iii-v until the additional base station is operating at a target power level.
Preferably, prior to the installation of an additional base station, the existing network is characterised and optimised. A new frequency plan may be deployed for the existing network such that there are spare radio channels available for the additional base station to use.
The incremental increase in signal level can be predetermined. The signals for which the power levels are monitored can be characteristic signals such as control channel signals. The process of increasing signal power levels can also include an adjustment of antenna down-tilt The normal operating signal power level can be determined by a balance in traffic load across new and old base stations.
In accordance with a further aspect, the invention thus minimises the loss of service to existing customers during a capacity upgrade for a fixed wireless access system. The concept of reducing service outage is typically regarded by operators as a higher priority than minimising the time taken or the amount of resource used in order to provide seamless service transition when a subscriber""s antenna is re-oriented. The present invention thereby enables a minimum of visits by personnel in connection with this process, for RSS repointing.
In accordance with a still further aspect of the invention there is provided a software analysis tool operable to determine, in a fixed wireless access network which is subject to installation of an additional base station, a number of subscribers which should be repainted to said additional base station, said tool having an input operable to receive measurement data from a radio element manager, analysis means operable to determine those subscriber stations which can be repointed, an output to provide a list of subscriber stations which can be repainted, estimator means operable to estimate the effect of the repainting of subscriber stations and to determine which subscriber means should be repointed.
In accordance with a further aspect of the invention, there is provided a computer program stored on a computer readable medium and arranged to control a software analysis tool operable to determine those subscriber stations which can be repointed, an output to provide a list of subscriber stations which can be repointed, estimator means operable to estimate the effect of the repointing of subscriber stations and to determine which subscriber means should be repointed.
In accordance with a still further aspect of the invention, there is provided a fixed wireless access network which has increased in capacity by the installation of an additional base station within an existing cellular deployment plan, wherein, in the commissioning of the additional base station, the signal power levels were incrementally increased from an initial level too low to cause interference with subscriber receivers within an area of coverage of the additional base station until normal operating signal power level is achieved; wherein, for each power level, at each subscriber premises within the area of coverage of the additional base station, the signal level from additional base station was compared relative to a base station with which the subscriber has been communicating; and repointing the subscriber if the signal level is improved.
In accordance with a further aspect of the invention, there is provided a fixed wireless access network which has increased in capacity arising from the installation of an additional base station within an existing cellular plan; wherein, in the commissioning of the additional base station, the signal power levels were incrementally increased from an initial level too low to cause interference with subscriber receivers within an area of coverage of the additional base station until normal operating signal power level is achieved; wherein, for each power level, at each subscriber premises within the area of coverage of the additional base station, the signal level from additional base station was compared relative to a base station with which the subscriber has been communicating; and repointing the subscriber if the signal level is improved